Mycobacterium lepraemurium (agent of rat leprosy and for 69 years a famous "obligate intracellular parasite") initiates growth in Nakamura's (1972) system and serves as an interim model for Mycobacterium leprae and other "host-dependent" organisms. Ultrasensitive determinations of ATP (developed specifically to investigate the growth potential and functional biomass of unwashed host grown and "host dependent" microbes) quantitate the declining energy status of incubated type 1 (host grown) cells during the physiologic eclipse, the time required for type 11 (in vitro adapted) cells to generate energy at the rates used during growth in animal hosts and thereafter the expansions of bacterial biomass. OBJECTIVES: To use M. lepraemurium cells under controlled oxidation reduction potentials while: 1. Defining and supplying factors that overcome the cessation of growth after 6 weeks. Checking the significance of evident progress by raising the incubation temperature and, as soon as progress warrants, by trials with M. leprae. 2. Defining conditions and factors that greatly reduce the profound physiologic eclipse of type 1 (in vivo grown) cells and, thus, to facilitate the cultivation of Mycobacterium leprae and other "host dependent" organisms.